The compounds of the formula I are known. They are intermediates in the process for the preparation of the biologically active 2-alkoxy-carbonylamino-benzimidazole derivatives. There has been known only two processes for the preparation of the isolated product so far.
According to the working example of Hungarian Patent Specification No. 177,418 bis[(2-methoxycarbonyl)-amino-benzimidazole-5(6)-yl]-disulphide is reduced with sodium metal in liquid ammonia, then the thiol is liberated from the sodium salt thiol-compound remaining after the evaporation of the ammonia. The disadvantage of the above process is that it is quite difficult to carry out from an industrial point of view, expensive equipment should be employed and as both sodium metal and liquid ammonia are dangerous to handle, special security measures have to be followed to. The melting point of the product prepared according to the example is 260.degree. to 262.degree. C.
According to Hungarian Patent Specification No. 182,782 the product is prepared from the hydrochloric salt of a sulphonic chloride falling under the scope of the formula II with metals in the presence of water and mineral acid. Tin, zinc and iron were exemplified as reducing agents. In all cases the reduction resulted in the corresponding metal salt of the thiol of the formula I, which precipitated from the given reaction mixture and thus it could be isolated. According to the only example relating to the preparation of the isolated thiol, the tin salt of the thiol obtained as a product of the reduction was decomposed by hot sodium carbonate solution, the insoluble tin hydroxide was removed by filtration and the thiol was recovered from the filtrate. This method is lengthly and quite great losses occur as the thiol oxidizes to the corresponding disulphide in a quite short period. A further problem is that it is very difficult to purify the product from the tin contamination, it can only be obtained in several stages and leads to losses. Therefore according to the above specification in order to avoid the difficulties accompanying the separation of the thiol, the metal salts were further reacted. Thus the metal contaminations could be eliminated in a further stage of the reaction.
As to the metals used in the above specification we have to note that the highest yield could have been achieved when tim was used. However, the price of tin is high, therefore this process is uneconomic. Zinc has a high ability to form complexes and this results in different problems, while the yield is not satisfactory when iron is employed.
Considering these difficulties it is strange that the above specification does not mention that aluminum could be used.
By improving the known methods a simple, economic process was seeked for the preparation of the thiole of the formula I, which enables the industrial scale production of the said thiol in an easy manner and results in a pure product.